Anionic polymerization systems can be used to produce trans-1,4-polybutadiene (TPBD) with good molecular weight control being achieved. In such anionic polymerizations there is typically an inverse relationship between the catalyst level utilized and the molecular weight attained. An anionic polymerization system for producing TPBD is disclosed in U.S. Pat. No. 4,225,690. The catalyst system disclosed therein is based on a dialkylmagnesium compound which is activated with a potassium alkoxide. However, such catalyst systems have not proven to be commercially successful.
TPBD is normally prepared utilizing transition metal catalysts or rare earth catalysts. The synthesis of TPBD with transition metal catalysts is described by J. Boor Jr., "Ziegler-Natta Catalysts and Polymerizations," Academic Press, New York, 1979, Chapters 5-6. The synthesis of TPBD with rare earth catalysts is described by D. K. Jenkins, Polymer, 26, 147 (1985). However, molecular weight control is difficult to achieve with such transition metal or rare earth catalysts and monomer conversions are often very modest.
Japanese Patent Application No. 67187-1967 discloses a catalyst system and technique for synthesizing TPBD consisting of 75 to 80 percent trans-1,4-structure and 20 to 25 percent 1,2-structure. The catalyst system described by this reference consists of a cobalt compound having a cobalt organic acid salt or organic ligand, an organoaluminum compound and phenol or naphthol. Gel formation is a serious problem that is frequently encountered when this three-component catalyst system is utilized in the synthesis of TPBD. Gel formation is normally encountered in cases where the catalyst system described in Japanese Patent Application No. 67187-1967 is utilized in continuous polymerizations.
U.S. Pat. No. 5,089,574 is based upon the finding that carbon disulfide will act as a gel inhibitor in conjunction with three component catalyst systems which contain an organocobalt compound, an organoaluminum compound and a para-alkyl-substituted phenol. U.S. Pat. No. 5,089,574 also indicates that conversions can be substantially improved by utilizing para-alkyl-substituted phenols which contain from about 12 to about 26 carbon atoms and which preferably contain from about 6 to about 20 carbon atoms.
U.S. Pat. No. 5,089,574 more specifically reveals a process for synthesizing trans-1,4-polybutadiene in a continuous process which comprises continuously charging 1,3-butadiene monomer, an organocobalt compound, an organoaluminum compound, a para-substituted phenol, carbon disulfide and an organic solvent into a reaction zone; allowing the 1,3-butadiene monomer to polymerize in said reaction zone to form the trans-1,4-polybutadiene; and continuously withdrawing the trans-1,4-polybutadiene from said reaction zone.
The techniques described in U.S. Pat. No. 5,089,574 are very useful in improving conversions and reducing gel formation. However, its teachings do not describe a technique for controlling the molecular weight of the TPBD being synthesized. In many applications, it would be desirable for the TPBD produced to have a lower molecular weight. There is, accordingly, a need to control the molecular weight of the TPBD produced with such Ziegler-Natta catalyst systems.
U.S. Pat. No. 5,448,002 discloses that dialkyl sulfoxides, diaryl sulfoxides and dialkaryl sulfoxides act as molecular weight regulators when utilized in conjunction with cobalt-based catalyst systems in the polymerization of 1,3-butadiene monomer into TPBD. U.S. Pat. No. 5,448,002 reports that the molecular weight of the TPBD produced decreases with increasing levels of the dialkyl sulfoxide, diaryl sulfoxide or dialkaryl sulfoxide present as a molecular weight regulator.
U.S. Pat. No. 5,448,002 specifically discloses a process for the synthesis of trans-1,4-polybutadiene which comprises polymerizing 1,3-butadiene monomer under solution polymerization conditions in the presence of at least one sulfoxide compound selected from the group consisting of dialkyl sulfoxides, diaryl sulfoxides and dialkaryl sulfoxides as a molecular weight regulator and in the presence of a catalyst system which includes an organocobalt compound, an organoaluminum compound and a para-alkyl-substituted phenol.
The presence of residual cobalt in TPBD made with cobalt-based catalyst systems is not desirable. This is because the residual cobalt can lead to polymer instability during storage. This is a particular problem in cases where the TPBD is stored in a "hot-house" prior to usage which is a standard procedure in many industries, such as the tire industry. In any case, higher levels of residual cobalt in the TPBD lead to worse problems with polymer instability. For this reason, it would be highly desirable to reduce the level of cobalt needed in catalyst systems which are used in the synthesis of TPBD. Reducing the level of cobalt needed is, of course, also desirable from a cost standpoint since cobalt compounds are relatively expensive.
Unfortunately, carbon disulfide is typically required as a gel-reducing agent in the synthesis of TPBD with cobalt-based catalyst systems. This is particularly true in the case of continuous polymerization systems. However, the presence of carbon disulfide in such systems reduces the level of catalyst activity and generally makes it necessary to increase the level of cobalt in the catalyst system. Thus, in cases where carbon disulfide is required for gel control, the level of cobalt needed is further increased.
By utilizing the techniques disclosed in U.S. Pat. No. 5,834,573, trans-1,4-polybutadiene having a trans-isomer content within the range of about 82 percent to about 87 percent can be synthesized continuously to a high level of conversion utilizing a low level of a highly active cobalt-based catalyst system. The trans-1,4-polybutadiene made with the cobalt-based catalyst system of U.S. Pat. No. 5,834,573 also typically has a dilute solution viscosity in the range of about 1.4 to about 2.4 which is acceptable for use in tire applications and is essentially gel-free.
U.S. Pat. No. 5,834,573 more specifically discloses a process for synthesizing trans-1,4-polybutadiene in a continuous process which comprises continuously charging 1,3-butadiene monomer, cobalt (III) acetylacetonate, an organoaluminum compound, a para-alkyl-substituted phenol and an organic solvent into a reaction zone, wherein the cobalt (III) acetylacetonate is mixed with the para-alkyl-substituted phenol prior to being charged into the reaction zone; allowing the 1,3-butadiene monomer to polymerize in said reaction zone to form the trans-1,4-polybutadiene; and continuously withdrawing the trans-1,4-polybutadiene from said reaction zone. In practicing the process of this invention, it is preferred for the molar ratio of the para-substituted phenol to the cobalt (III) acetylacetonate to be within the range of about 12:1 to about 16:1 and for the molar ratio of the organoaluminum compound to the cobalt (III) acetylacetonate to be within the range of about 16:1 to about 24:1.
TPBD can be blended with various rubbers to improve performance characteristics and green strength. Since TPBD can undergo strain induced crystallization it is particularly valuable for use in tire tread rubber stocks. However, TPBD is typically a thermoplastic resin at room temperature by virtue of its high level of crystallinity. This makes it necessary to heat conventional TPBD to an elevated temperature before it can be incorporated into rubber compounds. Such a technique is the subject of U.S. Pat. No. 5,854,351 which is based upon the discovery that TPBD which contains a processing oil can be rapidly heated by radio frequency electromagnetic radiation.
U.S. Pat. No. 5,854,351 more specifically discloses a technique for mixing a trans-1,4-polybutadiene with at least one rubbery polymer which comprises: (1) heating the trans-1,4-polybutadiene to a temperature which is within the range of 105.degree. F. (41.degree. C.) to 200.degree. F. (93.degree. C.) by exposing it to electromagnetic radiation having a frequency in the range of about 2 MHz to about 80 MHz, wherein the trans-1,4-polybutadiene is oil-extended with at least 10 phr of a processing oil; and (2) mixing the trans-1,4-polybutadiene with said rubbery polymer before any portion of the trans-1,4-polybutadiene cools to a temperature below 104.degree. F. (41.degree. C.).